The present invention is directed generally to the detection of radial position, and in particular to the use of reactance sensors, to indicate the radial position of the rotary elements of rotational devices.
Control of rotational electromechanical devices, such as electrical motors, typically requires knowledge of the position and/or speed of their rotors, and may further require information about the position of their axes of rotation. There are several ways to determine such parameters. First, the position of the rotor may be determined by an array of photo-transistors and a special shutter coupled to the rotor shaft, or by using Hall-effect sensors. Such systems are described in T. Kenjo, Electrical Motors and Their Controls, Oxford University Press (1994), pp 176 and following. Second, the speed informative signal may be obtained by using a small permanent magnet tachometer generator, attached to the shaft, or by using magnetic or optical sensors for generating pulses for each angular increment of the rotor. Such systems are described in W. Leonhard, Control of Electrical Drives, 2nd ed., Springer Verlag (2001), pp 420 and following. Third, a resolver may be used to determine the position of the rotor by a two-phase (sine/cosine) signal at a carrier frequency modulated sinusoidally by the rotation of the rotor. Such a system is described in J. R. Hendershot, Jr. and T. Miller, Design of Brushless Permanent-Magnet Motors, Magna Physics Publishing (1994), pp 1-19. All these methods require precise mechanical placement of sensors, or galvanic contact between moving parts.
Conventionally, the moveable element of a rotary device is the informative element that indicates rotational (angular) position; i.e., the information signal (an electrical signal) is generated on the moveable element. It is therefore necessary to have some means for transferring the information signal from this moveable element to external processing circuitry. This is usually accomplished by the use of rings and brushes, flexible connectors, and so on. The use of brushes can introduce noise into the information signal. Brushless solutions exist, but they suffer from low signal to noise ratios, and can be mechanically cumbersome. More significantly, brushes create problems with reliability and require constant maintenance. It is highly desirable to form and deliver signals to and from the rotating parts of mechanical or electromechanical devices without the use of mechanical or galvanic contact and a complex sensor supporting system.
Conventional motor drive technology using mechanical bearings suffers from many limitations. For example, mechanical bearings require lubrication, which can lead to high maintenance cost. Additionally, mechanical bearings tend to wear out and need to be replaced. As motor drive technology evolves, magnetic bearings and bearingless drives have been introduced to reduce the need for bearing lubrication and bearing replacement. Such systems are described in A. Chiba et al., Magnetic Bearings and Bearingless Drives, Newnes, Elsevier, (2005). FIG. 1 is a diagram illustrating a conventional motor system 100 including magnetic bearings. The motor 104 is located between electromagnetic bearings 102 and 106. Each electromagnetic bearing 102 and 106 have coils to generate radial forces in two perpendicular radial axes (e.g., X-Y coordinates shown in the figure). A feedback control mechanism is used to maintain the axis of rotation at the center of the stator core. Accordingly, the X-Y position of the axis of rotation must be provided to the control system to control the strength of the magnetic forces generated by the bearings 102, 106 in order to maintain the proper positioning of the axis of rotation.
In conventional magnetic bearings and bearingless drives systems, Cartesian coordinates are usually used to control the radial position of the axis of rotation. As shown in FIG. 1, electromagnetic bearing 102 is controlled in two radial axis coordinates x1 and y1, and electromagnetic bearing 106 is controlled in two radial axis coordinates x2 and y2. Even though this technique may be adequate in certain applications, there are many limitations as discussed below.
Accordingly, it is desirable to have improved methods and systems for sensing the radial displacement of a rotating element in an electromechanical device, and more specifically in devices having magnetic bearings and bearingless drives.